The present invention relates to a displacement converting apparatus in which a mechanical displacement proportional to a load change which is proportional to a quantity of a measured value such as pressure difference, pressure or liquid level is detected in terms of a variation of capacitance, and the capacitance variation is converted into a corresponding electric signal which is transmitted to a receiving apparatus. More particularly, the invention relates to a displacement converting apparatus in which a pair of capacitors, the capacitances of which differentially change or one of the capacitances of which changes in response to a mechanical displacement, are used to produce the sum of and the difference between the capacitances of the capacitors. The sum and the difference are used to derive the amount of (capacitance difference)/(capacitance sum) as a ratio of the change of the capacitances, and then the ratio is converted into an electrical signal proportional thereto.
In a capacitor type displacement detector (a so-called .DELTA.C cell), a diaphragm for detecting a load change proportional, for example, to a pressure difference .DELTA.P is disposed at the middle position between electrodes, and a capacitance C.sub.1 between the diaphragm and one electrode and a capacitance C.sub.2 between the diaphragm and the other electrode are obtained as follows: EQU C.sub.1 =.epsilon.A/(d-.DELTA.d) and EQU C.sub.2 =.epsilon.A/(d+.DELTA.d),
where .epsilon.: dieleectric factor, A: electrode area, d: distance between electrode and diaphragm, and .DELTA.d: amount of diaphragm displacement. The sum and difference of these two capacitances are given as follows: EQU C.sub.1 +C.sub.2 =.epsilon.A.multidot.2d/[d.sup.2 -(.DELTA.d).sup.2 ] EQU C.sub.1 -C.sub.2 =.epsilon.A.multidot.2.DELTA.d/[d.sup.2 -(.DELTA.d).sup.2 ].
Therefore, the following equation is obtained. EQU (C.sub.1 -C.sub.2)/(C.sub.1 +C.sub.2)=.DELTA.d/d.alpha..DELTA.P
As seen from the above equation, a pressure difference .DELTA.P may be detected by producing an electric signal proportional to (C.sub.1 -C.sub.2)/(C.sub.1 +C.sub.2).
In a known capacitor type displacement converter disclosed in Japanese Patent Application Publication No. 16,696/1978 published on June 2, 1978, a DC voltage Vi proportional to the difference between a pair of capacitances C.sub.1 and C.sub.2 and a DC voltage Vr proportional to the sum of these capacitances C.sub.1 and C.sub.2 are detected. Then, (C.sub.1 -C.sub.2)/(C.sub.1 +C.sub.2) is obtained by performing the division Vi/Vr.
In this displacement converter, DC currents detected by rectifying and smoothing AC currents proportional to the variations of capacitances C.sub.1 and C.sub.2 are applied to a differential amplifier to control the value of an AC voltage applied to the capacitances in a manner such that the sum of these DC currents is made equal to a reference DC current. In this situation, it is required to employ a stable reference voltage element in order to secure stably the amount of the reference DC current, and it is further required to add a constant current circuit to this displacement converter so that a current flowing in the reference voltage element is maintained at a constant amount. As a result, there are disadvantages in that the circuit configuration thereof is complicated, and causes of error and trouble are increased because of the complicated arrangement of the circuit. Accordingly, it has been desired but difficult to overcome these disadvantages.
Further, as described above, the displacement converter having a pair of capacitors which have a common electrode moved by mechanical displacement and the capacitances C.sub.1 and C.sub.2 of which are varied differentially so as to indicate the amount of the mechanical displacement by the ratio of the sum and the difference of these capacitances C.sub.1 and C.sub.2, has also a disadvantage in that it is difficult to impose the movable electrode for varying the capacitances C.sub.1 and C.sub.2 on a heavy load such as a pressure gauge, a load cell or the displacer of a floating type liquid level gauge.